1. Field of the Invention
The present invention relates in general to a wavelength division multiplexing (WDM) optical network and, in particular, to an all-optical 3R regenerator (AO3R) and method for using the AO3R to retime, reshape and retransmit an optical signal in the WDM optical network.
2. Description of Related Art
WDM optical networks and in particular the optical signals traveling therein are distorted by a variety of transmission impairments. These transmission impairments can be induced by factors such as accumulated noise from optical amplifiers, waveform distortion, and nonlinear interaction in optical transmission fibers. Because, the WDM optical networks and optical signals are adversely affected by transmission impairments some type of optical signal regeneration scheme must be applied along the transmission path. Reference is made to the WDM optical network 100 shown in FIG. 1 to describe several traditional devices used to regenerate optical signals.
The WDM optical network 100 basically includes a series of transmitters 102 coupled to inputs of a multiplexer 104 which has an output coupled to one end of a transmission path 106. The other end of the transmission path 106 is coupled to an input of a demultiplexer 108 which has outputs coupled to a series of receivers 110. Depending on the length of the transmission path 106 there can be located therein one or more amplifiers 112 (only two shown) and one or more repeaters 114 (only one shown). The amplifiers 112 and repeaters 114 are used to compensate for the different transmission impairments that adversely affect the optical signal 116a. In particular, the amplifiers 112 shown as erbium-doped fiber amplifiers (EDFAs) are used to amplify the optical signals 116a and 116b. And, the repeater 114 which can be an electrical regenerator (O/E/O regenerator) 114a or an AO3R 114b is used to retime, reshape, retransmit the optical signal 116a as optical signal 116b. The repeater 114 can also be an all-optical 2R regenerator (AO2R) 114c that reshapes and retransmits the optical signal 116a but does not retime the optical signal 116a. Thus, the AO2R 114c has limited applications.
The O/E/O regenerator 114a includes opto-electronic circuits and electronic circuits that convert the optical signal 116a into an electrical signal that is retimed and reshaped in the electrical domain. The retimed and reshaped electrical signal is then converted back into an optical signal 116b and retransmitted by the traditional O/E/O regenerator 114a. An example a traditional O/E/O regenerator 114a is briefly discussed below with respect to FIG. 2.
Referring to FIG. 2, there is illustrated a block diagram of the basic components of an exemplary traditional O/E/O regenerator 114a. The O/E/O regenerator 114a includes a receiver 200 and a transmitter 230. The receiver 200 includes a photo diode 202 (e.g., PIN or APD) that converts the optical signal 116a to an electrical signal 204. The electrical signal 204 is amplified by an amplifier 206 and then divided and inputted into a clock recovery device (CDR) 208 and a phase comparator 210. The phase comparator 210 compares the amplified electrical signal 204 to an electrical signal 212 generated by the CDR 208 and outputs a retimed electrical signal 214 (shown as data (D) signal). The retimed electrical signal 214 is then divided and inputted into a low pass filter (LPF) 216 and the transmitter 230. The low pass filter 216 filters the retimed electrical signal 214 and outputs an averaged electrical signal 218. A voltage controlled oscillator (VCO) 220 receives the averaged electrical signal 218 and outputs a clock signal 222 (shown as clock (C) signal). The clock signal 222 is then divided by a power divider 224 and inputted into the CDR 208 and the transmitter 230. Thus, a feedback loop which includes the phase comparator 210, LPF 216, VCO 220, power divider 224 and CDR 208 is used to retime the electrical signal 204 and output the retimed electrical signal 214 (D signal).
The transmitter 230 receives the retimed electrical signal 214 (D signal) and the clock signal 222 (C signal) from the receiver 200. The transmitter 230 includes a flip-flop circuit (F/F) 232 that compares the retimed electrical signal 214 and the clock signal 222 to another clock signal 234 generated by clock 236. The F/F 232 outputs a regenerated data signal 238 to a laser 240. The laser 240 receives the regenerated data signal 238 and outputs a retimed, reshaped optical signal 116b that is transmitted from the transmitter 230 onto the transmission path 106 of the WDM optical network 100. There are a number of disadvantages associated with the O/E/O regenerator 114a. First, the O/E/O regenerator 114a is made from elaborate, cumbersome and expensive opto-electronic circuits and electronic circuits. Secondly, the O/E/O regenerator 114a requires and consumes a lot of power.
The AO3R 114b is expected to replace the O/E/O regenerator 114a, because the AO3R 114b is less expensive and requires less power than the O/E/O regenerator 114a. In addition, the AO3R 114b can directly process the optical signal 116a in the optical domain without converting the optical signal 116a into an electrical signal as required by the O/E/O regenerator 114a. However, traditional AO3Rs 114b are known by those skilled in the art to have very complicated and elaborate clock recovery schemes that attempt to recapture the clock signal from a data stream in the optical signal 116a. The AO3Rs need to recapture the clock signal of the optical signal 116a to properly output a retimed optical signal 116b. Examples of three traditional AO3Rs 114b have been described in the following documents which are incorporated by reference herein:                G. Raybon et al. “20 Gbit/s All-Optical Regeneration and Wavelength Conversion Using SOA Based Interferometers”, Lucent Technologies, 3 pages.        T. Otani et al. “40-Gb/s Optical 3R Regenerator Using Electroabsorption Modulators for Optical Networks”, Journal of Lightwave Technology, Vol. 20, No. 2, pages 195–200, February 2002.        J. Nakagawa et al. “All-Optical 3R Regeneration Technique Using Injection-Locking In Gain-Switched DFB-LD”, Electronics Letters, Vol. 37, No. 4, pages 231–232, Nov. 28, 2000.        
Accordingly, there is a need for an AO3R that has a clock recovery scheme that can recapture a clock signal from a data stream of a received optical signal in a manner that is more simplistic and effective than the elaborate clock recovery schemes used in traditional AO3Rs. This need and other needs are satisfied by the AO3R and method of the present invention.